To improve high temperature stability and to give better physical and electrical properties over amine cured epoxy resin systems, it has been the general practice in the epoxy technological field to use anhydride curing agents with epoxy resins, particularly for high voltage insulation applications. Most epoxy-anhydride formulations require elevated-temperature cures, and for most commercial applications it is necessary to add some material to speed the rate of cure. Consequently, a considerable amount of effort has been devoted in recent years to develop a perfect catalyst or accelerator for curing epoxy resins, especially those used for high voltage coil insulation, i.e., over about 7,000 volts. In high voltage coils, only an absolute minimum of voids can be tolerated in the resinous insulation. Therefore, the applied resin impregnating composition must be extremely fluid, solventless, and capable of a very fast gel, so that resin will not easily drain from a coil during curing.
The properties desired of such a catalyst or accelerator are: it should be inexpensive and readily available; it should gel the epoxy resin system, preferably at times below about 60 minutes at about 175.degree. C.; it should be completely soluble with the epoxy resin-anhydride system at all temperatures; the initial viscosity of the catalyzed resin system should be below about 350 cps. at 25.degree. C.; the preferred storage life of the catalyzed resin system should be over about 150 days at 25.degree. C., i.e., the viscosity should remain below about 1,000 cps. during that period; it should not adversely affect the mechanical properties of the cured resin system; after cure, the resin system preferably should have power factor values of below about 15% at 150.degree. C.
Several latent catalysts have appeared on the commercial scene in recent years. Among these are quaternary ammonium halides such as benzyltrimethyl-ammonium chloride, stannous octoate, "extra-coordinate" siliconate salts, triethanolamine borate, triethanolamine titanate and various other metal chelates. However, these materials failed to meet all of the above described requirements and have been rejected.
Smith, in U.S. Pat. No. 4,137,275, taught the use of a selected metal acetylacetonate, as a latent catalyst for a solventless, highly fluid, resinous epoxy-anhydride impregnating composition. The Smith materials provided most of the above mentioned desired properties. However, while the selected acetylacetonate, such as chromium or manganese acetylacetonate, combined superior gel times with good electrical properties, these latent catalysts had storage stabilities of from 160 to 200 days at 25.degree. C. While these pot life values are extremely good, even better storage stability is desirable for commercial applications.